(i) Field of the Invention
The present invention relates to a digital photographic instrument. More specifically, it relates to a method for adjusting the focus of a digital photographic instrument.
(ii) Description of the Related Art
In recent years, a camera is often installed in portable information devices such as a cellular phone and a PDA. Cameras used for such a purpose are provided in the market as camera modules equipped with a small-sized lens and an image sensor such as a CCD or CMOS. Initially, camera modules with a low resolution have been merely installed. However, recently, some of portable information devices are equipped with camera modules having a resolution of higher than 1 megapixel. It is almost certain that camera modules having a higher resolution will be installed in portable information devices in the future.
Meanwhile, a lens must be focused on a subject to take its clear picture. That is, the distance between the lens and an image sensor must be adjusted by the distance between a camera and the subject so that light having passed through the lens connects an image accurately on the image sensor. As a focus adjusting method, in a camera module equipped with an image sensor having a low resolution, a fixed focus method designed such that a lens can be focused over a wide range without moving by setting a great depth of field in advance is often used. However, since the focus must be adjusted more precisely as the resolution of the image sensor becomes high, it is desirable that the distance between the lens and the image sensor be adjusted aggressively by the distance between the camera and the subject.
In the field of general cameras and digital cameras that are devices designed specifically for photographing, an autofocusing function which focuses a lens on a subject automatically has been developed. The autofocusing function is a function of determining an appropriate focus evaluation value while moving the position of a lens gradually and taking a lens position at which the best focus evaluation value has been obtained as a lens position at which the optimum focus is obtained. As a method of determining the evaluation value, there can be named a phase difference detection method which has been used for film single-lens reflex cameras and a contrast method which is used for digital cameras.
To focus a lens on a subject precisely, it is desirable that a focus evaluation value be obtained at a number of lens positions by narrowing the distance between positions between which a lens is moved. However, this requires a long time to focus the lens on a subject and a large amount of power to move the lens. Accordingly, several proposals have been made to shorten time to focus a lens on a subject in a short time.
Japanese Patent Laid-Open Publication No. 2002-287012 describes an invention characterized by determining a focus evaluation value while moving the position of a lens and finding a focused focal position quickly by increasing the moving speed of the lens when the focus evaluation value is larger than or equal to a given threshold and decreasing the moving speed when the focus evaluation value is smaller than the threshold.
Japanese Patent Laid-Open Publication No. 2002-48967 describes an invention characterized by determining a focus evaluation value while moving the position of a lens and finding a focused focal position quickly by changing the width of the moving step of the lens according to the size of the focus evaluation value.
However, even with these conventional techniques, time to focus a lens on a subject is still not sufficient in some cases. In particular, some of camera-equipped cellular phones and PDAs take a photograph in a single-action mode and must complete control of focusing more quickly than cameras which take a photograph in a double-action mode.
The double-action mode is a mode in which preparations for photographing are made when a shutter button is pressed halfway and a photograph is taken when the shutter button is pressed ally the way. The preparations for photographing include automatic exposure control, autofocusing, and white balancing. Since a user generally takes a photograph after completion of the preparations for photographing, it can be said that cameras can take a sufficient time for the preparations for photographing.
In contrast, the single-action mode is a mode in which preparations for photographing and photographing are carried out at the press of a shutter button. If time between press of the shutter button and actual photographing is too long, a photographer and/or a subject move(s) and an out-of-focus picture is liable to be produced, and operational feeling is also unsatisfactory. Accordingly, it is necessary to complete the preparations for photographing quickly. The requirement for focusing speed in the single-action mode is more stringent than that in the double-action mode.